Field of the Invention
The present invention pertains generally to the field of medical anesthesia and more particularly relates to improvements in resistance syringes used for accurately positioning an epidural needle in preparation for injection of anesthetic into the epidural space of the spinal cord.
Epidural anesthesia involves the injection of a suitable liquid anesthetic substance into the epidural space which surrounds the dura mater which in turn surrounds the spinal cord proper. The epidural space is defined between the ligamentum flavum on the posterior or back side of the spinal cord, and the anterior longitudinal ligament on the anterior or frontal side of the spinal cord. These flexible but tough ligaments interconnect the bony vertebrae which enclose and protect the spinal cord and spinal canal.
A dose of a suitable anesthetic such as lidocaine or bupivacaine, by way of example, produces a regional nerve block suitable for surgical procedures to be performed on portions of the anatomy affected by the nerve block, primarily but not limited to portions of the anatomy below the level of the spinal column at which the anesthetic is injected.
In the preferred midline technique the epidural needle passes through the supraspinous, interspinous and ligamentum flavum structures before entering the epidural space. Insertion of the needle into the epidural space is complicated by the lack of feedback as to the position of the needle tip, coupled with the imperative need to avoid puncturing the dura mater which surrounds the spinal cord, since there is potential for catastrophic trauma to the spinal cord with the epidural needle. Extreme caution must therefore be exercised in the positioning of the needle tip, which must pierce through the tough, resilient, leather-like ligamentum flavum and then stop immediately within the narrow epidural space, short of puncturing the dura mater.
The needle must be moved through the ligamentum flavum very slowly and in a carefully controlled fashion. At the same time, pressure is applied to the plunger of the attached syringe which is filled either with air or saline solution. The object is to continuously test for loss of resistance to injection, experienced when the needle lumen enters the epidural space after clearing the ligamentum flavum. This loss of resistance is experienced by little if any resistance to injected air or fluid, and a negative aspiration test then indicates that the needle lumen is properly positioned in the epidural space.
In order to appreciate the contribution being made by the present invention, it is important to understand the demands placed upon the anesthesiologist's dexterity by this procedure. It is of critical importance that the needle traverse the ligamentum flavum in a carefully measured and controlled manner. Typically, this is achieved by applying resistance to the advancing needle with the anesthesiologist's non-dominant hand (the left-hand if the anesthesiologist is right-handed) while the dominant hand applies pressure to the plunger to test for resistance to injection while at the same time slowly advances the needle. Variations of this technique may be adopted according to personal preference, for example the needle may be advanced continuously while pressure on the syringe barrel is also maintained continuously to test for resistance. In the alternative, the needle is advanced in very small increments, e.g. 1 millimeter, testing for resistance to injection after each advance.
The difficulty of correctly positioning the needle lumen in the epidural space has spurred many attempts to develop methods and devices for detecting and indicating correct needle placement. These expedients have generally exploited the low resistance to injection and subatmospheric pressure characteristic of the epidural space. One such technique involves placement of a drop of saline solution on the open hub of a epidural needle. The drop will be "sucked-in" as the needle lumen enters the epidural space where, for reasons not well understood, prevails sub-atmospheric pressure. Other means used for this purpose include capillary attachments with fluid indicators developed by Odom, or inflated balloons by MacIntosh, which deflate upon entering the epidural space. It is also known to use spring loading devices to facilitate the loss of resistance phenomena which occur as the epidural needle passes from the dense ligamentum flavum into the lesser resistance of the epidural space.
It is an object of this invention to provide a simple, reliable, inexpensive disposable attachment to a conventional syringe, in particular for use with syringes of the so-called resistance type which are specifically designed for low barrel friction to facilitate kinesthetic sensing of changes in resistance to injection as the epidural needle passes through various anatomical structures.
A continuing need exists for an aid which will ease the demands placed on the anesthesiologist's dexterity in performing this procedure without resort to esoteric mechanisms, unfamiliar devices which require special handling, are difficult to sterilize, and generally unnecessarily expand the already over-crowded medical armamentarium.
The device of this invention is an attachment to conventional syringes which are well-known and familiar to any anesthesiologist. The attachment is low cost, very simple in construction, reliable and easy to use with minimal or no training, and in no way impairs the sterile condition of the syringe when attached to the same. Further, given that the device is an attachment to conventional syringes already used for anesthesia procedures, the added expense is very small yet affording a substantial reduction in the difficulty of epidural anesthesia procedures, with a consequent increase in safety of the procedure.
Because of these qualities, the present invention is intended to overcome the difficulties and disadvantages associated with the many previous attempts to facilitate administration of administering this challenging form of anesthesia. Given the potential for irreparable, catastrophic injury if the needle is mishandled, any aid which will increase the confidence of the anesthesiologist in performing epidural injections is of great value.